In approximately 1992, weldable RF connectors were introduced to the hermetic packaging industry as a substitute for solder-in feedthroughs that were being installed into aluminum microwave electronic packaging. The solder feedthrough system includes a low thermal expansion glass-to-metal-seal with an electroplated metal ferrule that is fixed within an electroplated receiving hole in the side wall of an aluminum electronic housing via a wetted solder joint. Due to the difference in coefficients of thermal expansion between the glass-to-metal-seal and the aluminum housing, these solder joints are inherently unreliable when subjected to multiple thermal cycles.
Over the last fifteen-years there have been many different types of weldable feedthroughs/connectors produced and although they increase the hermetic reliability of an aluminum package, which was the primary design goal of this product, the weldable feedthroughs and connectors are not without flaws.
When an RF coaxial feedthrough or connector is installed in an electronic package it may be desirable to mount it in a way that produces a short continuous path for the ground signal. The primary signal runs down the center wire in a coaxial cable and the ground signal runs down the shielded jacket outside the dielectric. For high frequency applications, microwave and higher, it may be important that these two signals run at the same pace as each other to keep the signal in-phase. If the primary signal runs ahead of the ground signal, the combined signal will become out-of-phase. An out-of-phase signal may exhibit noise and static and generally be of poor quality.
Any time there is a physical change in the signal paths of the coaxial cable there is a challenge to keep the signal “clean” and in-phase. For example, when a cable is attached to a connector and the connector is mounted on or within a sidewall of an electronic housing, physical change may occur that can disrupt the RF signal if not designed properly. The primary signal path is typically always carried on the center conductor of the cable and has a straight path through the connector/feedthrough into a circuit board, for example. The design challenge comes from trying to make the ground signal travel the same distance as the primary signal when it is running through the connector and into the electronic package. Any disruption or increase in ground path distance relative to the primary signal path distance may cause unwanted noise in the transition from the cable to the circuit board.
FIG. 1 illustrates a prior art coaxial connector 100 in perspective view and in an exploded relationship with a mounting hole (hole detail) 120. The coaxial connector 100 has a dielectric body 110 surrounding the center conductor 115, which isolates it from the metallic components of the coaxial connector 100. In the prior art, to ensure a good connection between the ground signal, which travels near the surface of the dielectric and the metallic substrate to which the coaxial connector 100 is mounted, four spring clips 105 are positioned to facilitate the transfer of the ground signal to the metallic substrate 150 in which the hole detail is provided. In the prior art, the hole detail is sized to provide a slip fit for the coaxial connector 100 so that it may easily fit into the hole detail.
During installation, the coaxial connector 100 is placed into the hole detail and held in place, for example with tweezers, during a welding process, using, for example, a laser welder, in which the welding beam progresses circularly around the circumference of the boundary between the hole detail and the substrate, forming welds 130 which form a hermetic seal between the coaxial connector 100 and the substrate 150.
The solder-in feedthroughs described above, were particularly unsuitable for providing hermetic seals because of the solder fatigue, which results from thermal recycling. When used in avionics, for example, the solder joint might range in temperature from 80° C., when an aircraft was located on a landing strip in the middle of a desert, to 65° C. when the same aircraft was located at an altitude of 70,000 feet. After a certain number of thermal cycles, the solder-in feedthroughs may fail and the hermetic seal may be lost.
The weldable connectors improved the thermal recycling properties and substantially addressed the problems of thermal recycling. However, the weldable connectors produced other problems. When using connectors at high frequencies, for example, between 2 Ghz and 100 Ghz, it may be important that the ground signal path be the same length as the path through the center conductor of a coaxial transmission line. At these high frequencies, even a slight variation in path length may result in substantial interference.
Further, the installation process for the weldable connectors for use at high frequencies is very sensitive. There is for example the need to keep the connectors centered within the hole detail of the housing to have a reproducible impedance. Further, the technique of holding the connector in place with tweezers still permits wiggle room between the connector and the slip fit sized hole detail.
When performing a laser weld operation, the laser beam weld results in displacement of metal on the hot side of the connector while the opposite side of the connector remains cool. This can cause the axis of symmetry through the connector to become off normal or tilted with respect to the substrate, which can disturb and create gaps in the RF ground plane. Specifically, any tilt in the coaxial connector 100 shown in FIG. 1, may result in one of the ground signal pins 105 lifting away from the substrate and thus provide a less than adequate connection. This could disturb and create gaps in the RF ground plane. It may be desirable that the coaxial connector 100 remain concentric within the hole detail of the housing after welding to function properly and maintain a matched impedance. The ground signal may desirably stay close to the dielectric to maintain a (typically) 50 ohm impedance. Particularly with short coaxial connectors 100, the welding process may result in sufficient tilt so that there is, instead of 360 degrees of contact between the coaxial connector 100 and the contact for the ground springs 105, there may be as little as 180 degrees of contact and very poor concentricity.
Still further, removal or replacement of the coaxial connector 100 from the hole detail 120 typically involves removing the welds 130. Removal of the welds may damage both the hole detail 120 and the coaxial connector 100. Thus, there is an increased cost with removal or replacement of the coaxial cable 100 as the hole detail may have to be replaced, or have remaining burrs removed.